feat: add LinkedIn OAuth 2.0 / OIDC authentication emulator

[DaleWebb]

Summary

Adds a new @emulators/linkedin package that emulates LinkedIn's Sign In with LinkedIn using OpenID Connect surface, matching exactly what Better Auth's LinkedIn provider expects.

Endpoints

Method	Path	Description
GET	/.well-known/openid-configuration	OIDC discovery document
GET	/oauth2/v3/certs	JWKS (stub)
GET	/oauth/v2/authorization	Authorization page (user picker)
POST	/oauth/v2/authorization/callback	Auth code generation + redirect
POST	/oauth/v2/accessToken	Token exchange (authorization_code + refresh_token)
GET	/v2/userinfo	LinkedIn profile (sub, name, given_name, family_name, picture, locale, email, email_verified)
POST	/oauth/v2/revoke	Token revocation

Features

• Strict client validation when oauth_clients are seeded (consistent with all other emulators)
• PKCE support (plain + S256)
• client_secret_basic (Authorization header) support
• HS256 id_token signing
• Seed config for users and oauth_clients
• Registered in CLI as linkedin service

Structure

Mirrors @emulators/microsoft exactly:

packages/@emulators/linkedin/
  src/
    entities.ts          # LinkedInUser, LinkedInOAuthClient
    helpers.ts           # generateSub()
    store.ts             # getLinkedInStore()
    routes/oauth.ts      # All OAuth/OIDC routes
    index.ts             # Plugin, seedFromConfig, exports
    __tests__/linkedin.test.ts  # 15 tests

Seed config

linkedin:
  users:
    - email: testuser@linkedin.com
      name: Test User
  oauth_clients:
    - client_id: example-linkedin-client-id
      client_secret: example-linkedin-client-secret
      name: My LinkedIn App
      redirect_uris:
        - http://localhost:3000/api/auth/callback/linkedin

Tests

15 tests covering: OIDC discovery, JWKS, authorization page, client validation, full OAuth flow with id_token verification, refresh tokens, single-use auth codes, unsupported grant types, userinfo, token revocation, client_secret_basic auth, and seed config.

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